A magnetic random access memory (MRAM) collects attention as a nonvolatile memory in which a high-speed write operation can be carried out and the number of times of rewrite is large.
FIG. 1 shows a typical MRAM disclosed in U.S. Pat. No. 5,640,343. The MRAM contains a memory cell array in which memory cells 101 are arranged in a matrix. The memory cell 101 is interposed between a word line 102 extending in an X axis direction (word line direction), and a bit line 103 extending in a Y axis direction (bit line direction).
As shown in FIGS. 2A and 2B, each memory cell 101 contains a magneto-resistance element (spin valve). The magneto-resistance element has a pinned layer 104 and a free layer 105 which are made of ferromagnetic material, and a non-magnetic spacer layer 106 interposed between the pinned layer 104 and the free layer 105. The pinned layer 104 may be connected to a diode 107 in order to apply a desired bias to the memory cell 101. When the non-magnetic spacer layer 106 is a very thin insulating layer, the magneto-resistance element is sometimes called MTJ (Magnetic Tunnel Junction). Both the pinned layer 104 and the free layer 105 which are formed of the ferromagnetic material have spontaneous magnetizations (residual magnetization). The direction of the spontaneous magnetization of the pinned layer 104 is fixed in the +X direction, and the direction of the spontaneous magnetization of the free layer 105 can be freely reversed in the +X direction or a −X direction. Anisotropy is given to the free layer 105, and the free layer 105 is formed in such a manner that the direction of the spontaneous magnetization is easily directed to the X-axis direction.
The memory cell 101 stores a 1-bit data as the direction of the spontaneous magnetization of the free layer 105. The memory cell 101 can take two states of a “parallel” state in which the direction of the spontaneous magnetization of the pinned layer 104 is identical to that of the spontaneous magnetization of the free layer 105, and an “anti-parallel” state in which the direction of the spontaneous magnetization of the pinned layer 104 is opposite to that of the spontaneous magnetization of the free layer 105. The memory cell 101 stores the 1-bit data regarding one of the “parallel” state and the “anti-parallel” state as “0” and the other as “1”.
The data is read from the memory cell 101 by detecting the change of resistance of the memory cell 101 due to magnetic resistance effect. The directions of the spontaneous magnetizations of the pinned layer 104 and free layer 105 affects the resistance of the memory cell 101 through the magnetic resistance effect. When the direction of the spontaneous magnetization of the pinned layer 104 is parallel to that of the spontaneous magnetization of the free layer 105, the resistance of the memory cell 101 is a first value R (FIG. 2B). When the direction of the spontaneous magnetization of the pinned layer 104 is anti-parallel to that of the spontaneous magnetization of the free layer 105, the resistance is a second value R+ΔR (FIG. 2A). Therefore, the data stored in the memory cell can be detected by detecting the resistance of the memory cell 101.
The write operation of the data is carried out in a selected memory cell among the plurality of memory cells 101 through the following process. With reference to FIG. 1, one of the word lines 102 relating to the selected memory cell is selected as a selected word line, and one of the bit lines 103 relating to the selected memory cell is selected as a selected bit line. Currents flow through the selected word line and the selected bit line, and the spontaneous magnetization of the free layer 105 of the selected memory cell is directed to a desired direction based on a synthetic magnetic field of a magnetic field generated by the selected word line and a magnetic field generated by the selected bit line.
The role of the magnetic field generated by the selected bit line is different from that of the magnetic field generated by the selected word line. The magnetic field generated by the selected bit line determines the direction of the spontaneous magnetization of the free layer 105 of the selected memory cell. When the current flows through the selected bit line extending in the Y-axis direction, the magnetic field is generated in the +X direction or the −X direction, and the direction of the spontaneous magnetization of the free layer 105 of the selected memory cell is changed in the +X direction or the −X direction by the magnetic field. A magnetic field is not applied to the memory cell 101 which is not connected to the selected bit line and the direction of the spontaneous magnetization of the free layer 105 is not reversed. Therefore, the direction of the spontaneous magnetization of the free layer 105 in each memory cell with which the selected bit line does not intersect is saved.
On the other hand, the magnetic field generated by the selected word line makes it easy that the direction of the spontaneous magnetization of the free layer 105 of the selected memory cell is reversed. The direction of the magnetic field generated by the selected word line is the +Y direction or a −Y direction, and is a direction perpendicular to the direction in which the spontaneous magnetization of the free layer 105 can is directed. Therefore, the magnetic field generated by the selected word line does not determine the direction of the spontaneous magnetization of the free layer 105 directly. However, the ferromagnetic material of the free layer 105 is made easy in the reversion of the direction of the spontaneous magnetization by applying the magnetic field into the direction perpendicular to the direction of the spontaneous magnetization. The magnetic field generated by the selected word line is directed into the direction perpendicular to the direction of the spontaneous magnetization of the free layer 105, and therefore, the coercive force of the free layer 105 of the selected memory cell is reduced.
On the other hand, the magnetic field generated by the selected word line is not applied to non-selected memory cells of the memory cells 101 connected to the selected bit line. Therefore, the coercive force of the free layer 105 of the non-selected memory cell is not reduced. This means that the difference in the coercive force of the free layer 105 exists between the selected memory cell and the non-selected memory cell. The data can be selectively written in the selected memory cell based on the difference of the coercive force between the selected memory cell and non-selected memory cell.
FIGS. 3A to 3C show the principle of selective data write into the selected memory cell described above. The coercive force of the free layer 105 shows a characteristic called an asteroid curve (magnetization reversing magnetic field curve). This functions as a threshold function. When a magnetic field in the outside region of the asteroid curve is applied, the magnetic field exceeds the coercive force, and therefore, the direction of the spontaneous magnetization of the free layer 105 is reversed. The asteroid curves shown in FIGS. 3A to 3C show that the direction of the spontaneous magnetization of the free layer 105 is most easily reversed when a synthetic magnetic field directing the direction of 45 degrees to both the X axis and the Y axis directions is applied to the free layer 105 by the selected bit line and the selected word line.
The currents flowing through the selected bit line and the selected word line are selected in such a manner that the synthetic magnetic field of the magnetic fields generated by the selected bit line and the selected word line is located in the outside region of the asteroid curve, and each of the magnetic fields generated by the selected bit line and the selected word line is independently located in the outside region of the asteroid curve. The selection of the currents flowing through the selected bit line and the selected word line permits the selective write operation of the data into the selected memory cell.
One of the technical problems in the above-mentioned write operation of the data in the MRAM is power consumption. As described above, the write operation of the data into the MRAM is carried out by reversing the direction of the spontaneous magnetization by using the magnetic fields generated by the currents. For this reason, a relatively large current are required. The large currents increase the power consumption in the write operation of the data into the MRAM.
An MRAM in which the power consumption in the write operation of the data is reduced is disclosed in Japanese Laid Open Patent Application (JP-P2001-273760A). The MRAM is provided with a high permeability layer formed of high permeability material on the upper surface or lower surface of a write current path. The high permeability layer centralizes the magnetic field generated by the current path on the memory cell, and effectively suppresses the power consumption in the write operation of data.
Another MRAM in which the power consumption in the write operation of data is reduced is disclosed in Japanese Laid Open Patent Application (JP-P2002-110938A). The MRAM of this conventional example is provided with a magnetic film formed on the upper surface and side surfaces of the word line, and a magnetic film formed on the bottom surface and side surfaces of the bit line. The magnetic film is formed of high saturation magnetization soft magnetic material or metal-non-metal nanogranular material. The magnetic film allows the magnetic field to effectively act on the memory cell, and suppresses the power consumption at the time of the write operation of data effectively.
Another technical problem in the write operation of the data in the MRAM is selectivity of the selected memory cell. As described above, the write operation in the selected memory cell is realized in the MRAM by optimally selecting the magnitudes of the write currents flowing through the selected bit line and the selected word line according to the shape of the asteroid curve of the free layer. Therefore, the write currents flowing through the selected bit line and the selected word line, and the asteroid curve of the free layer must be adjusted in a high precision. However, it is hard to avoid changes of the write currents flowing through the selected bit line and the selected word line and the asteroid curve because of a variation in a manufacturing process and a variation in use environment of the MRAM. The changes degrade the selectivity of the selected memory cell, and a malfunction may be caused in which an undesired data is written in the non-selected memory cell in the write operation of data.
In conjunction with the above description, a magnetic random access memory is disclosed in Japanese Laid Open Patent Application (JP-P2002-8367A). In this conventional example, the magnetic random access memory has a plurality of sense lines and a plurality of word lines, and a unit memory cell is arranged at each of the intersections of the sense lines and the word lines in a two-dimensional array. The unit memory cell has a series connection of a cell selection switch provided with a function of a voltage drop element and a magneto-resistance element. Each of the sense lines is provided with a capacitance section, and electric charge stored in the capacitance section is sequentially discharged through the sense line, the cell selection switch, and the magneto-resistance element. Thus, a magnetic holding state of the magneto-resistance element is distinguished based on the voltage change of the capacitance section caused due to the discharge.